Chapter 14, Problem 26P

Interpretation Introduction

Interpretation:

The difference in the stabilities ofcycloheptatrienone andcyclopentadienone needs to be explained and the structure of Diels–Alder adduct of cyclopentadienoneneeds to be drawn.

Concept introduction:

In accordance with theHuckel’s rule, aromatic compounds contain a planar ring and have a conjugated pi system with $(4n+2)\pi$ electrons, where $n=0,1,2,3,\mathrm{...}$

Anaromatic compound contains conjugated pi bonds.

Antiaromatic compounds contain $(4n)\pi$ electrons. They are very unstable and are highly reactive.

Diels-Alder reaction is an addition reaction of a conjugated diene and an alkene to form a cyclohexene derivative. It is a single step reaction.

The general reaction of Diels–Alder is as follows:

Diels Alder reaction is between a conjugate diene and a reactant containing double bond (dienophile) to form the product and the product is called adduct.

Diels Alder reactions are highly stereospecific. The configuration, if dienophile is retained in the product and the reaction, is syn addition.

The dienes react with dienophiles in cis forms rather than trans forms.

Endo and exo refer to the orientation of dienophile and its electron withdrawing group, when it reacts with a diene in Diels–Alder reaction.

The reaction with orientation of electron-withdrawing group of dienophiles under the $\pi$ orbitals of the diene, is Endo.

The reaction with orientation of electron-withdrawing group of dienophiles away from the $\pi$ orbitals of the diene is Exo

Endo is favored in the transition state of Diels–Alder reaction because of its lower energy.

If a compound is stable, it has lower energy and if a compound is unstable, it has higher energy.